A primary-side phase-shift DC-DC converter combined with a full-bridge inverter and a rectifier circuit is typically configured as follows. The full-bridge inverter is made up of the following circuit. The circuit is configured with two legs: a first leg and a second leg connected in parallel, each leg has two switching elements constituting arms connected in series, each arm has a capacitor connected in parallel with the switching element and a diode connected in anti-parallel therewith; and both ends of the parallel-connected legs serve as the input terminals, and points in the connections between the arms of the first leg and between the arms of the second leg serve as the output terminals.
The full-bridge inverter converts a DC voltage supplied across the input terminals into a high-frequency AC voltage, to output the high-frequency AC voltage to the primary side of a transformer connected to the output terminals. The secondary side of the transformer is connected to the rectifier circuit, and the rectifier circuit rectifies the high-frequency AC voltage output from the transformer. High-frequency components in the output of the rectifier circuit are removed by an output smoothing filter and the resultant DC voltage is supplied to a load.
The full-bridge inverter thus configured typically controls its transmission power using phase shift control. The phase shift control controls the transmission power by varying the overlap angle between the first leg and the second leg. Further, the phase shift control enables soft switching operation to reduce switching loss by connecting a reactor in series with the primary winding of the transformer. In a state of the second leg being delayed in phase with respect to the first leg, the two switching elements constituting the first leg operate the soft switching with zero-voltage and zero-current switching (ZV&ZCS) turn-on utilizing current continuity in the reactor and with zero-voltage switching (ZVS) turn-off utilizing the tangent of the rising voltage of the capacitor connected in parallel with each switching element; on the other hand, the two switching elements constituting the second leg operate the soft switching with ZVS turn-on and ZVS turn-off utilizing a resonance phenomenon caused by the reactor and the capacitor connected with each switching element.
However, a circulation period is needed to enable the first leg to turn on with ZV&ZCS. The circulation period is a period during which a current is conducted between the positive-side arms or between the negative-side arms in the primary side and no power is transmitted. The circulating current, since it makes no contribution to the transmission power to the load, results in reactive power, thus raising a problem of conduction loss caused by the circulating current in the switching elements, the diodes, and the transformer. Moreover, setting the circulation period reduces the power transmission period and limits the transmission power from the transformer. Although setting the transformer turns ratio to high is a conceivable technique as a countermeasure for enabling power to be transmitted to the secondary side even in a narrow power-transmission period, the voltage applied across the secondary-side diodes increases, thus leading to the need for using an element having a higher withstand voltage. Since diodes generally show a positive correlation between the withstand voltage and the conduction characteristic, using a diode having a high withstand voltage poses a problem of increasing conduction loss.
Patent Document 1 discloses a method for overcoming these problems. The method disclosed in Patent Document 1 is that a capacitor is connected between the rectifier circuit and the output smoothing filter to advance the current passing through the transformer with respect to the voltage applied thereto and to increase time product of the voltage applied to the transformer and the current passing therethrough, whereby the transmission power from the transformer is improved.